The present invention relates to a seismic sensor, in particular to a seismic sensor that incorporates a geophone.
A geophone is a well-known seismic sensor (or seismic receiver) that is in widespread use in the field of seismic surveying. Typically, an array of geophones will be distributed around a source of seismic energy. Energy emitted from the seismic source is reflected by geological structures within the earth, and the reflected energy is received at the geophones. From an analysis of the energy reflected to each particular geophone it is possible to derive information about the geological structure of the earth.
A geophone incorporates a moving coil. Seismic energy incident on the geophone induces vibrations of the coil, and an electrical output signal is derived from the vibrations of the coil. In order to ensure accurate operation of the geophone the moving coil must be mounted with its axis vertical, or at least within xc2x110xc2x0 of the vertical.
One type of prior art seismic sensor comprises a geophone that is packaged in a housing. A spike is provided on the exterior of the housing, and this spike extends in a direction substantially parallel to the axis of the moving coil. Such a seismic sensor is used by pushing the spike into the ground, to secure the sensor in position. This ensures good coupling between the geophone and the ground, and if the spike is driven hard into firm ground the axis of the geophone is unlikely to move.
One disadvantage of this known seismic sensor is that, as noted above, it must be disposed such that the axis of the moving coil of the geophone is within xc2x110xc2x0 of the vertical. This means that after each geophone has been planted in the ground, it must be checked to ensure that the axis of the moving coil is within 10xc2x0 from the vertical, and this is time-consuming where a large array of sensors is being used. If the orientation of the geophones is not checked, and it should happen that some of them are mounted with the axis of the moving coil at more than 10xc2x0 from the vertical axis, then the quality of the obtained seismic data will be degraded. A further disadvantage of this prior art seismic sensor is that it is very difficult to deploy automatically, since the processes of planting the sensor in the earth and checking its orientation is difficult to automate.
An alternative prior art seismic sensor that comprises a geophone has the geophone moveably mounted in a housing. The geophone is mounted on gimbals that incorporate low-friction ball bearings. Because the geophone is mounted on gimbals, it can orientate itself such that the axis of the moving coil is vertical regardless of the orientation of the housing.
Although this prior art seismic sensor solves the problem of mounting the sensor such that the axis of the coil is substantially vertical, it is not without its own disadvantages. In particular, external noise sources can generate unwanted oscillations of the geophone, and this will produce noise in the output signal from the geophone and so degrade the quality of the seismic data obtained. It is possible to introduce a viscous liquid into the housing to dampen oscillations of the geophone induced by external noise, but this does not provide a complete solution since it will not prevent oscillation of the geophone. A further disadvantage of this type of seismic sensor is that they are expensive, owing to the need to provide the complex gimbal mechanism.
The present invention provides a seismic sensor comprising: a housing; a geophone rotatably mounted within the housing; and a locking means for releasably preventing movement of the geophone relative to the housing.
A seismic sensor according to the present invention has two states: locked and unlocked. In the unlocked state, the geophone can move, for example rotate, within the housing, in order to orientate itself with the axis of its coil substantially vertical. Once the geophone is correctly orientated, the sensor can be locked. In its locked state, movement of the geophone relative to the housing is prevented. Since the geophone is locked relative to the housing, a source of external noise will not cause unwanted oscillations of the geophone, so that the sensor can be used to obtain high quality seismic data.
In a preferred embodiment of the invention, the locking means comprising a locking material disposed within the housing, the locking material being solid at the normal operating temperature of the sensor. In order to unlock the sensor, it is necessary only to heat the locking material to its melting point; once the locking material melts, the geophone will be able to rotate within the housing. Once the geophone is correctly oriented, the sensor is locked simply by allowing the locking material to cool below its melting point.
In an alternative preferred embodiment, the locking means is moveable between a first position in which it exerts a locking force on the geophone so as to prevent movement of the geophone relative to the housing and a second position. The sensor is locked or unlocked by moving the locking means into the first position or the second position respectively.
Other preferred features of the present invention are set out in the dependent claims, to which attention is directed.